It's even worse with the younger generation. They can't even use Swedish idioms so they switch to English. For example, they might know and use "You're on thin ice" but don't know what "Ute på hal is" eller "Ute på tunn is" means. If I say "det ligger en hund begraven här" they will look at me funnily and not understand what I mean. However, if I say "Something smells fishy" they will know.
People older than me used even funnier idioms that no one uses anymore. For example "Du ler förnöjt som en katt som nyss svalt en kanariefågel."
I don't know what to do about it, if anything. :/ Undoubtedly, it is easier if everyone speaks the same language. But it is also much less interesting and much less diverse.